Saggy Bikes: Static vs Dynamic hardtail geo- Mtbr.com
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  1. #1
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    Saggy Bikes: Static vs Dynamic hardtail geo

    I need this forum's collective wisdom. When hardtails are weighted, the front fork sags, steepening the HTA and STA, and increasing the reach.

    On a full suspension, the front and rear sags (more or less) evenly, preserving the reach and angles of the bike.

    Lets start with a blatant generalization. For 29in full suspension bikes:
    • XC HTA: 67.5-69deg (Scott Spark, Intense Sniper, Santa Cruz Blur)
    • Trail HTA: 66-67deg (Pivot 429, SC tallboy, Stumpy)


    Now lets look at the SC Chameleon as an example of a trail hardtail. I modeled the bike straight off the SC geo chart, with an un-sagged fork:
    Saggy Bikes: Static vs Dynamic hardtail geo-29er-chameleon-no-sag.jpg

    Now here is the bike with 25% sag:
    Saggy Bikes: Static vs Dynamic hardtail geo-29er-chameleon-25-sag.jpg

    When accounting for sag, the reach jumps from 460mm to 475mm and the HTA steepens to 68.7deg. Suddenly the large chameleon becomes XL'ish and the HTA changes from trail to XC territory.

    A survey of "trail" hardtails, both custom and production reveals that most have HTA's steeper than 67deg. When sagged, this puts them solidly in XC geo category.

    So here are some questions:
    • Lets say I love how my FS 29er fits and corners. If I copy the geo on a HT frame, accounting for sag, will it feel more or less the same?
    • Or, is there something about how a HT rides that make's the HTA less relevant than on a FS?
    • Under actual riding (not just static sag tests), do the forks actually sag 25%?
    • What has been your experience with the handling of 66-67deg sagged (65-66deg static) HTA on a trail hardtail?


    Ultimately, HT's will always ride differently than a full suspension. They will feel twitchy and chaotic regardless of the geo. However, when I ride my FS, I do enjoy the calmer steering and the way it encourages me to lean the bike more to corner. I also like that the wheel is further out (placebo effect?). I think that has a lot to do with the sagged steering geo.

    Bonus can of worms: What do you guys think of the new trend short 41-44mm offset forks coupled with 1-deg slacker HTA on a hardtail frame? Has anyone tried this yet?

  2. #2
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    Many manufacturers list the sagged numbers, but there's not any sort of standard there.

    If you copy your FS geometry, roughly, accounting for sag, then yes, you'll get generally similar steering characteristics. There is nothing magic about a hardtail that will make it steer differently.

    My personal feeling is that there's no point in going super slack on hardtails, since you just end up in the "front end writing checks the rear can't cash" situation. But people love them their super slack super long bikes these days, so maybe I'm just old and grouchy.

    -Walt

  3. #3
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    My question would be, aside from preferring the more confidence inspiring cornering feel of your full suspension, do you want your hardtail to ride like a bike you already own? The most enjoyable part of having both types of bikes for me is that each bike has a unique feel. I will ride my hardtail differently through the same technical section than I will ride my full suspension and that adds to the enjoyable variety of having more than one bike. When you're feeling loose and rowdy hop on the HT but when you want smooth and stable grab the fully!

  4. #4
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    Possibly irrelevant...

    I have a bike that I am pretty sure I bent (the downtube). To improve/correct the steering, I run the fork near the top of the the travel. It is sagged maybe 10% on a 120mm fork. It also has 5 tokens in it. It never bottoms out. It handles great until the roots get bigger than ~3" or so.
    I have tried it with a shorter rigid fork (quite a twitchy handfull), and I have tried it all up and down the 120mm fork. I would hazard a guess that says steering trail might be the number that matters most. A hardtail would typically have a shorter rear end and overall WB, so it's bound to be a little less stable than an FS bike no matter what. Of course, if you're building from scratch, your chainstays can be 30" long.

    And if I recall correctly, Trek went with more fork offset ("G2") on 29ers to speed up the steering a bit, right? This goes back to the trail figure. Slack it out, and now it needs another tweak to get a decent trail figure (???) (I always thought 51mm was too far anyway). I've had a 500 ATC, 47mm offset fork on my ~69* rigid 29er for 7 years (it would normally run a 120-140mm fork). I thought of having someone cold bend it for more offset, but it's so close to perfect there's no point.

    -F
    It's never easier - you just go faster.

  5. #5
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    Quote Originally Posted by natzoo View Post
    • What has been your experience with the handling of 66-67deg sagged (65-66deg static) HTA on a trail hardtail?



    That's my happy place for hardtails.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  6. #6
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    Saggy Bikes: Static vs Dynamic hardtail geo

    Nothing but positive experiences going slacker on my current hardtail. My bike is unique in that I only run a 120 fork, my reach is exactly right for my body and it has a higher than currently fashionable B.B. height. I’ve never felt that my front end is “writing cheques my rear end can’t cash” a sensation folks often lament. Having the front wheel further ahead of me increased my rear wheel traction, stabilized the front wheel at speed, prevents over the bar crashes and optimizes fork operation. I would not hesitate to it exactly again the same way and would consider slacker in consideration of the other geometry figures but not in isolation.

    Edit: Hardtail is 65.5 at 20% sag with a 41mm offset fork.

    On the suspension bike I have I’ve run 65.5 static and felt it could be fine but if the bike is overly sagged at the rear that 65.5 gets pretty slack and I do a lot of climbing so I experience that super slack often. So again looking at that figure in isolation of other geometry figures (or suspension operation) is not terribly helpful.

  7. #7
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    Some brands take sagged geo into account and design accordingly. Cotic used to only give their sagged numbers, while Ragley mentioned in their marketing that they design to match the numbers of a fs bike at sag. Whyte's designer has said that their T130 line derived from the 900 series of hardtails (not vice versa), and the head angle on the ht's is slacker so that they have the same geo at sag.

    However, head angle is not the only important measurement. A long wheelbase will make any bike more stable at speed, but a slack head angle is not the only way to increase the distance between the wheels. Mondraker for example have more conservative head angles but very generous reach numbers, and end up with huge wb's.

    My own hardtail size small has a 430mm reach and a 618mm top tube, but only a 66.5 deg. static head angle. The stability is there due to the long frame, and the fore/aft balance feels perfect both up and down. I contemplated getting an angleset and a longer airspring, but finally decided against it as I didn't want to disturb the balance between front and rear centers given that my chainstay length is fixed at 425mm.
    Last edited by justwan naride; 03-01-2019 at 04:29 AM.

  8. #8
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    Quote Originally Posted by justwan naride View Post
    Some brands take sagged geo into account and design accordingly. Cotic used to only give their sagged numbers, while Ragley mentioned in their marketing that they design to match the sagged numbers of a fs bike at sag. Whyte's designer has said that their T130 line derived from the 900 series of hardtails (not vice versa), and the head angle on the ht's is slacker so that they have the same geo at sag.

    However, head angle is not the only important measurement. A long wheelbase will make any bike more stable at speed, but a slack head angle is not the only way to increase the distance between the wheels. Mondraker for example have more conservative head angles but very generous reach numbers, and end up with huge wb's.

    My own hardtail size small has a 430mm reach and a 618mm top tube, but only a 66.5 deg. static head angle. The stability is there due to the long frame, and the fore/aft balance feels perfect both up and down. I contemplated getting an angleset and a longer airspring, but finally decided against it as I didn't want to disturb the balance between front and rear centers given that my chainstay length is fixed at 425mm.
    Agreed. Front-center is the number to pay attention to. Head angle, reach, trail, offset... bla bla... these are for describing how the steering feels. Not super important unless it's shit. Your looking at rear center is smart.

    It's interesting; I'm unusually tall, and i think unusually short people have more insightful things to say wrt bike fit than most folks who are closer to my size. Road bikes are a different thing. Y'all are getting ****ed.
    "Things that are complex are not useful, Things that are useful are simple."
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  9. #9
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    I'm currently having a custom full suspension frame designed that's based on my rigid bike. I absolutely love everything about the rigid, and hoping the builder can replicate the feeling with the addition of both front and rear suspension.

    I think in my case, fork weight will also slightly change the balance of the bike with the suspension fork being over 2 pounds heavier.

  10. #10
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    Quote Originally Posted by justwan naride View Post
    However, head angle is not the only important measurement. A long wheelbase will make any bike more stable at speed, but a slack head angle is not the only way to increase the distance between the wheels. Mondraker for example have more conservative head angles but very generous reach numbers, and end up with huge wb's.
    For sure.

    I think all the numbers deserve consideration in relation to each other. If there is one piece of advice I'd leave someone with that was wanting to get a custom bike built it is to consider all the numbers and how each will affect the other.

    Here's one example on my most recent bike. Being shorter and in particular with a short torso. I wanted more stability via longer wheelbase, more front centre (FC) etc. but I have limitations in seated cockpit that are unavoidable. I chose the shortest "normal" stem on the market and a lowish sweep bar, considered seated cockpit fit (with the available range of seat rail length.) first and foremost and designed around that. My reach ended up wherever it ended up but I still wanted more FC so what am I left with? Running a larger offset fork (minor gains) and/or running a slacker head angle. In this case I chose a slacker head angle.

  11. #11
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    Couldnt believe how this thread fitted me so well.

    My history is this:

    Im a short guy, 5'4"/ 163cm with short legs. But dont like the feeling of super short bikes, had a bad experience with a 2014 Small Specialized Camber bike that was way too short even for me.

    Today I have 2 Santa Cruz full sus bikes:

    - 2015 Nomad 3 Medium. I tried the Small Nomad 3 and felt it too short, thats why I bought the Medium. Also Id like to have a longer wheelbase and more stable feeling, since Id be riding this bike faster on more rough stuff.
    - 2017 Hightower Small (OG 135mm frame ,150mm fork). This one is for trail riding and even XC. Also my wife (who has same height as me but little longer legs) rides it too, so I bought it Small size, because Santa Cruz made the 2016+ bikes a little longer than the previous one and for the aplication I wanted felt right. Tried the medium one but the top tube lenght was too big for me.

    Some months ago I decided to build the hardtail that I wanted for ages.
    So after thinking a lot about the geometry I came up with something in between my full sus bikes.
    Im not a frame builder, so I order the frame with this geo:



    The geometry is very close to the Crhomag Surface but with a little steeper head angle, because Id like to behave also as a XC bike too if i wanted. Its based on a 500 A2C (120mm sagged fork), so If I use a 140mm fork will slack the head angle a bit (about 65.5) without hurting the sta too much (about 74). I will even try it with a 480mm A2C rigid fork to see what happens.

    I didnt built it yet, but will leave my impressions as soon as I do it.

    The big question is:

    I ve read some conflicting opinions regarding the seat tube angle.

    the current trend is very steep STA, wich is valid for full suspension bikes and I understand perfeclty why. But for hardtails, specially long travel ones, cant be too steep because the fork will sag a lot and make it even steeper.
    Peter Verdone (maybe himself could reply here since hes an active member) make his hardtails with very slack seat angle and says its more confortable like this.
    Than Chromag came with this new monster with 77 STA and 180mm fork.

    So, Im lost.

    I dont think my hardtail will ride like shit, since its almost a Chromag surface , but sometimes I regret if I shouldnt go with a steeper STA like 76 or 77.
    Last edited by TheLittle; 1 Week Ago at 06:01 AM.

  12. #12
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    Seat angle doesnt need to be steep to be comfortable. A more relaxed seat angle is good for traversing trail type riding. The Doctahawk is designed for what I call "winch and plummet" riding which is pretty much the de facto riding here on the coast. We crawl up steep pitches and then hang on back down. Pretty much doctrinally opposite to the riding that PVD has in his back yard. So pick what works best for where you are. Dont be tied to trends that aren't aimed at you. Most bike companies are only marketing companies and have only a tenuous understanding of how bikes can ride.

  13. #13
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    Quote Originally Posted by nouseforaname View Post
    Pretty much doctrinally opposite to the riding that PVD has in his back yard.
    Never been here?
    "Things that are complex are not useful, Things that are useful are simple."
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    Sure. I've not been in more than a couple of places in California. But I've not seen anything that even hinted at anything like Gargamel. And I think that if there was, we wouldn't have to have put up with such shitty bikes from mainstream companies for so long.

  15. #15
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    Quote Originally Posted by nouseforaname View Post
    Sure. I've not been in more than a couple of places in California. But I've not seen anything that even hinted at anything like Gargamel. And I think that if there was, we wouldn't have to have put up with such shitty bikes from mainstream companies for so long.
    LOL... yeah that's what i thought.

    FYI there's no shortage of local extended climbs where a poorly fitted bike has the rider perma-crunched for 45 minutes as they wheelie up the mountain. I haven't seen anything as awful as what we have available in 15 years of mtb traveling. We know. I can't speak for PVD, but for me seat angle, chainstay length, front center, and front end geometry all interact and it's a compromise. A 77* seat angle might be a good compromise in a few locations... but it's a stupid hill to die on.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  16. #16
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    I have pro-level legs and lungs, or at least close, and I don't wheelie out until I'm basically out of power/traction anyway. That's on boring 73 degree seat angles and VERY short chainstays.

    Obviously YMMV but not everyone needs a super steep STA. And for most riding (I love the term "winch and plummet" btw) there's not any advantage.

    As others have noted already, moving the rider forward when climbing is most useful for REALLY steep climbing on bikes that have VERY long front ends and have wheel weighting issues. If both of those apply (ie aggro bike at the 'shore) it's worth doing. If only one is at issue, it's a hard call. If neither, forget it.

    -Walt

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    Quote Originally Posted by nouseforaname View Post
    Sure. I've not been in more than a couple of places in California. But I've not seen anything that even hinted at anything like Gargamel. And I think that if there was, we wouldn't have to have put up with such shitty bikes from mainstream companies for so long.
    Ive being once in Gargamel. and sh*&*& that place is for crazy people hahaha. Pushed up most of it, and also pushed down on the scariest stuff.

    we have some climbs here in Rio that are 40 min with 17% average inclination, some short parts 30% to 35%. Can do most of it with my 74 ish seat angle bikes (nomad and higtower) but some times I have to push depending on how tired I am or how bad the grip is.

    so considering all this I think I could climb well with a 75 head angle hardtail, and wont have much compromisse pedaling on more flat stuff, like long rides with not much elevation changes. Ill keep 75° as it is.
    Last edited by TheLittle; 3 Days Ago at 06:40 AM.

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